In electron microscopy devices, such as a scanning electron microscope (SEM), an electron beam is sharply focused onto a specimen like a semiconductor wafer. A desired region on the specimen is scanned with the beam. The electron beam irradiation of the specimen produces secondary electrons, which are detected. The resulting signal can be displayed as an image.
An electron optical column is used in electron microscopy. Conventional electron optical columns for electron microscopy typically include an electron source with an electron emitter, such as in a Schottky emission gun or a field emission gun, for producing an electron beam. The electron beam may be used to produce a scanning probe or illuminate a sample or an aperture using a series of electron beam lenses, which may be magnetic or electrostatic. An electron optical column also typically includes an electrostatic pre-accelerator lens that focuses the electron beam and a series of lenses that refocuses and images the source aperture or sample onto the target.
An electron source with an electron emitter, such as in a Schottky emission gun or field emission gun, typically includes an electrode adjacent to the emitter, called an extractor electrode or extractor. The extractor may be configured to generate an electrostatic field at the emitter thereby causing electron emission and acceleration from the emitter into the rest of the electron source and eventually the electron optical column.
In previous extractor designs, the extractor bore sidewalls presented a large surface area to the primary electron beam. Such a large surface area generated a large number of secondary electrons. Secondary electrons interact strongly with the primary electron beam, preventing the beam from achieving a small spot size further down in the electron optical column.
A larger spot size can have negative impact to electron beam systems. This may be especially true if an electron source uses a magnetic lens for focusing the primary beam. The secondary electrons move more slowly than the primary electrons, and the magnetic field from the lens tends to trap them in the vicinity of the primary electron beam for a relatively long time.
An extractor with rounded edges was previously tested. The electron source, in one such case, used an electrostatic lens rather than magnetic lens. However, the manufacturing method produced a radius of curvature at the extractor upper surface of about 100 μm. This is in excess of a value that can effectively minimize the generation of secondary electrons.
Therefore, what is needed is an improved extractor that reduces secondary electron generation.